1. Field of the Invention
The present invention relates to a LCD driving circuit, and in particular to a DC/DC module of the LCD driving circuit.
2. The Related Arts
Typically, an active matrix display device comprises a row driver and a column driver to drive the dot arranged in a matrix form. It uses a liquid crystal display (LCD), a plasma display panel (PDP) and an organic light emitting diode display (OLED) as the active matrix display device.
In recent years, the liquid crystal display (LCD) has become more popular in a flat panel display. In addition to other applications, the flat panel display can also be used as television, computer monitor, personal digital assistant (PDA) and mobile phone. With the continuous improving research of the LCD, it has developed all kinds of methods to manufacture the LCD device. The most important is the improvement of the driving circuit of the liquid crystal display.
Typically, the LCD driving circuit input voltage is 12V, and the input voltage is converted into various voltages required for the LCD displaying through the DC/DC module/architecture. Nowadays, the voltages required for the LCD display comprise an upper limit output voltage (VAA) of the liquid crystal driver (about 15V), a lower limit voltage VBB of the liquid crystal driving output (grounded), VDD (it is typically 3.3V, which is the operating voltage of several IC's logic circuit such as EEPROM), the gate off-state voltage VGL of a thin film transistor (TFT) (typically about −6V), the gate on-state voltage VGH of TFT (typically about 33V).
FIG. 1 is a schematic diagram illustrating the DC/DC module of the LCD driving circuit according to the prior arts. Referring to FIG. 1, the prior arts usually use the DC/DC module as shown in FIG. 1 to convert the input voltage. The input voltage of the DC/DC module is 12V, and the input voltage is converted into various voltages required for the LCD displaying through the DC/DC module.
The 12V input voltage generates VAA, which is about 15V, through a boost circuit. A boost converter is a single-tube non-isolated DC-DC converter in which the output voltage is higher than the input voltage. The working principle of the boost circuit is shown in FIG. 2. Wherein, Q is a transistor, the driving voltage of which typically is pulse width modulation (PWM) signal; the signal period is Ts, the signal frequency is f=1/Ts, the on-state time is Ton, the off-state time is Toff, the period is Ts=Ton+Toff, and the duty cycle is Dy=Ton/Ts. The maximum duty cycle Dy of the boost circuit must be limited. Working under the condition of Dy=1 is not allowed. The inductor Lf is located at the input side, which is called boost inductor.
The 12V input voltage generates VDD, which is about 3.3V, through a BUCK circuit. A buck converter is a single-tube non-isolated DC-DC converter in which the output voltage being less than the input voltage. The working principle of the buck circuit is shown in FIG. 3. Wherein, Q is a transistor, the driving voltage of which typically is PWM signal, the signal period is Ts, and the duty cycle is Dy=Ton/Ts.
The 12V input voltage generates the gate off-state voltage VGL of the TFT, which is about −6V, through the negative charge pump, and the 12V input voltage generates the gate on-state voltage VGH of the TFT, which is about 33V, through the charge pump. The charge pump is also called as switched-capacitor voltage converter, which is a kind of the DC-DC (converter) stored the energy by a so-called “flying” or “pumping” capacitor (rather than the inductor or transformer). They can increase or decrease the input voltage and also generate the negative voltage. Wherein, the internal FET switch array controls charge and discharge of the flying capacitor in a certain way, so that the input voltage is multiplied or decreased in a certain factor (0.5, 2 or 3) to obtain the required output voltage. This specific adjusting process can ensure being up to 80% efficiency. Since the circuit is used for switching, the charge pump structure will generate a certain output ripple and electromagnetic interference (EMI). The working principle of the charge pump can be referred to FIG. 4, FIG. 4 (a) is the most simple circuit diagram of the positive charge pump, and FIG. 4 (b) is the corresponding input and output waveform. A brief description of the working principle is as follows, the voltage conversion can be achieved in two stages. In the first stage, the switches S1 and S2 are closed, and the switches S3 and S4 are cut off. The capacitor is charged until its value which is equal to the input voltage, that is UC1+−C1−=UC1=UIN. In the second stage, the switches S3 and S4 are turned off, and S1 and S2 are turn on. Because the voltage drop across the capacitor can not be changed immediately, the output voltage becomes twice the value of the input voltage, that is UOUT=UIN+UC1=2UIN. Using this method can achieve the double voltage. Usually, when the duty cycle of the switching signal is 50%, it can produce the best charge transfer efficiency. Of course, this is just a simple calculation, and the output voltage is not 2 UIN in reality. The negative charge pump principle is similar and not to be repeated here.
The defects in the prior art DC/DC architecture is: it needs boost circuit to boost voltage, in which the structure is complex and the hardware cost is higher. Furthermore, as the size of the panel increases, and gate driver on array (GOA) so on architecture, the output ability of the gate off-state voltage is required to be strong enough. However, the negative charge pump according to the prior art converts voltage by the capacitor. The carrying capacity is weaker, which can only maintain 150 mA output.